Three types of circulating fluidized bed exist which differ in the way in which reactor temperature is regulated, which temperature must be kept constant at a value close to 850.degree. C. in order to ensure effective desulfurization of the flue gases:.
the first type has panel heat exchangers installed inside the reactor (METALL-GESELLSCHAFT's French patent No. 2 323 101) and to maintain said temperature it adjusts the density of solid matter either by regulating primary and secondary air flow rates, or else by recycling combustion gases at avariable rate; however, as the power of the installation increases, it becomes necessary to extend the installation of said panel heat exchangers further and further down inside the reactor, thereby correspondingly increasing the risks of erosion;
the second type has external heat exchangers disposed on the external recirculation line for solid matter picked up at the outlet from the reactor by means of a separator (METALLGESELLSCHAFT's French patent No. 2 353 332); such external heat exchangers being installed at a distance from the reactor and thus requiring linking ductwork between the cyclone separator and the external heat exchanger, and between the external heat exchanger and the reactor, together with the necessary slopes and expansion joints; as the power of a reactor is increased, the heat exchange power of the tube walls of the reactor generally does not increase proportionally because of the limitation in the height of the reactor, and thus the power of the external heat exchangers increases more quickly, as does the number of such heat exchangers and their dimensions; this makes installation thereof even more difficult or even impossible and provides a limit at present on the electrical power that can be considered for use with this technology; and
the third type is that described by STEIN INDUSTRIE in its European patent application No. 91 401 041.8, and it has a decrease in the fluidization gas velocity inside the reactor on going past a bubbling bed installed at an intermediate level of the reactor; this velocity decrease is obtained by a large and quantified change in the cross-section of the reactor (ratio lying in the range 1.2 to 2) for the purpose of improving combustion by means of an increase in the amount of solid matter recirculated to the lower portion of the reactor; because a heat exchanger exists in said internal bubbling bed, this third type of reactor makes it possible to reduce the heat exchange power compared with that of the internal panels of the first type of circulating fluidized bed or with that of the external heat exchangers of the second type of circulating fluidized bed; however, in general, it does not make it possible to omit them in high power units.